The control of sea lice (Lepeophtheirus salmonis and Caligus elongatus) infestations in commercial salmon farming operations is still largely dependent on the use of chemical treatments (Roth M., Richards R. and Sommerville C. (1993) xe2x80x9cCurrent Practices In The Chemotherapeutic Control of Sea Lice Infestations: A Reviewxe2x80x9d Journal of Fish Diseases (16(1): 1-26). Outbreaks of these ectoparasitic copepods are currently treated by immersion bath treatments, with the organophosphates dichlorvos (Aquagard(copyright) Novartis) and Azamethiphos (Salmosan(copyright) Novartis), or hydrogen peroxide (Salartect(copyright) Brenntag, Paramove(copyright) Solvay-Interox) or the synthetic pyrethoids, cypermethrin (Excis(copyright) Vericore) and deltamethrin (Alphamax(copyright) Alpharma). Bath procedures are very labor intensive, costly and cause considerable stress to fish. Further, such treatments may not be feasible on exposed sites and during adverse weather conditions.
With the exception of cypermethrin, (Jakobsen P. J. and Holm J. C. (1990) xe2x80x9cPromising Test With New Compound Against Salmon Licexe2x80x9d Norsk Fiskeoppdrett. January, 16-18), bath treatments are only effective against pre-adult and adult stages of sea lice, allowing chalimus stages to survive and continue the cycle of infestation. Treatments are therefore indicated only when populations reach the pre-adult and adult phases, and thus must be repeated frequently for effective control. Resistance to the organophosphate dichlorvos has been identified in some populations of sea lice (Jones M. W., Sommerville C. S. and Wootten, R. (1992) xe2x80x9cReduced Sensitivity of the Salmon Louse, Lepeophtheirus salmonis, to the Organophosphate Dichlorvosxe2x80x9d Journal of Fish Diseases 15:197-202). Hydrogen peroxide may cause damage to the gills and its use is restricted in summer owing to its toxicity at higher water temperatures (Thomassen J. M. (1993) xe2x80x9cHydrogen peroxide as a Delousing Agent for Atlantic Salmonxe2x80x9d In: Pathogens of Wild and Farmed Salmon: Sea Lice (ed. by G. Boxshall and D. Defaye) Ellis Horwood Ltd. London).
A treatment that is effective against all parasitic stages of sea lice and other parasites which could be administered in feed, to avoid the disadvantages associated with bath applications, would be beneficial to the salmon industry. In-feed treatment allows medication during adverse weather conditions and on exposed sites and will permit simultaneous medication of all cages on a site and all sites in a loch system or single bay, thus reducing any cross infestation that may occur during the several days necessary to apply bath treatments to all cages on a site. The in-feed treatments currently available are the insect growth regulators, diflubenzuron (Lepsidon(copyright) Ewos) and teflubenzuron (Calicide(copyright) Nutreco) (Erdal J. I. (1997) xe2x80x9cNew Drug Treatment Hits Sea Lice When They are Most Vulnerablexe2x80x9d. Fish Farming International vol.24, No.2). Their mode of action is the inhibition of chitin synthesis (Horst M. N. and Walker A. N. (1996) xe2x80x9cBiochemical Effects of Diflubenzuron on Chitin Synthesis in the Post-molt blue crabxe2x80x9d (Callinectes sapidus) Journal of Crustacean Biology. 15: 401-408) and activity is therefore restricted to the moulting stages of sea lice.
The avermectins, produced by the culture of Streptomyces avermilitis, have highly potent anthelmintic and insecticidal properties. A chemically modified derivative, ivermectin (22,23-dihydroavermectin B1) was developed as a broad spectrum anti-parasitic for cattle, sheep, horses and pigs (Sutherland I. H. (1990) xe2x80x9cVeterinary Use of Ivermectinxe2x80x9d Acta Leidensia 59: 211-216) and has been marketed world-wide since 1981. Ivermectin has also been extensively used in the treatment of several human parasitoses (Ottesen E. A. and Campbell W. C. (1994) xe2x80x9cIvermectin in Human Medicinexe2x80x9d Journal of Antimicrobial Chemotherapy. 34(2): 195-203). Following the recognition of organophosphate resistance in sea lice (Jones M. W., Sommerville C. S. and Wootten, R. (1992) xe2x80x9cReduced Sensitivity of the Salmon Louse, Lepeophtheirus salmonis, to the Organophosphate Dichlorvosxe2x80x9d Journal of Fish Diseases 15:197-202), ivermectin was considered as an alternative therapy. In addition to its novel mode of action, a further advantage lay in its application as an in-feed medication. Although ivermectin has not received regulatory approval for use in salmon, it may be prescribed in the United Kingdom by veterinarians under the cascade procedure (Anonymous (1998) Amelia No.8 Veterinary Medicines Directorate. Woodham Lane, Newhaw, Addlestone, Surrey KT15 3NB), where authorised products fail to provide effective control of sea lice infestations. The use of ivermectin over several years has indicated that it exercises some control at the commonly adopted dose rate of 25 xcexcg kgxe2x88x921 biomass twice weekly (Rae G. H. (1996) xe2x80x9cGuidelines for the Use of Ivermectin Pre-Mix for Pigs to Treat Farmed Salmon For Sea Licexe2x80x9d Scottish Salmon Growers Association pamphlet). However, ivermectin has been found to be toxic at levels greater than 25 xcexcg kgxe2x88x921 biomass twice weekly (S. C. Johnson, et al., xe2x80x9cToxicity and Pathological Effects of Orally Administered Ivermectin In Atlantic, Chinook, and Coho Salmon and Steelhead Trout,xe2x80x9d Diseases of Aquatic Organisms. Vol. 17: 107-112 (1993).
Emamectin (4xe2x80x3-deoxy-4xe2x80x3 epimethylaminoavermectin B,) has been recently used for treating edible plant crops (Leibee G. L., Jansson, R. K., Nuessly, G and Taylor J. L. (1995) xe2x80x9cEfficacy of Emamectin Benzoate and Bacillus thuringensis at Controlling Diamondback Moth (Lepidoptera: Plutellidae) Populations On Cabbage in Floridaxe2x80x9d Florida Entomologist. 78(1): 82-96).
This invention provides a method of eliminating, reducing, or preventing parasites in a fish population, comprising feeding emamectin or a salt thereof to said fish population at a daily dose of 25 xcexcg to 400 xcexcg per kg of fish biomass per day for a period of 3-14 days.
In a further aspect, a kit for preparing a medicated fish feed for eliminating, reducing or preventing parasites in a fish population is provided, comprising a supply of emamectin or a salt thereof and printed instructions for feeding the emamectin or emamectin salt at a daily dose of 25 xcexcg to 400 xcexcg per kg of fish biomass per day for a period of 3-14 days.